The Reykjanes Ridge Experiment

Motivations: The Reykjanes Ridge, a major topographic feature of the North-Atlantic Ocean lying south of Iceland, is localized in a strategic place for the Meridional Overturning Cell (MOC) that is the primary mechanism for the meridional transport of heat, salt and carbon. It influences the main paths followed by the two limbs of the MOC and is the gate towards the deep convection areas for the warm water masses of the upper limb. However, the temporal mean and variability of the structure of the flow along and over the ridge and the prevailing dynamical regimes are still poorly characterized and understood. In addition, numerical models used both for climate scenarios (1° to 1/4°) and operational short-term forecasts(1/12°) present deficiencies in the subpolar gyre. This is partly due to a wrong representation of flow-topography interactions around the Reykjanes Ridge, which tends to bring too much warm and salty water to the Irminger and Labrador Seas. The model resolution is a likely source of problem as very high-resolution (1/36°) is necessary to resolve the relevant dynamical scale, the Rossby radius of deformation. Additionally, observation over the ridge of enhanced turbulent mixing, a key parameter that modifies water mass properties, suggests that deficiencies in the representation of vertical mixing may be an additional source of model biases. Indeed, turbulent diapycnal mixing is ten-to fifty-fold larger near the Reykjanes Ridge than in the neighboring Irminger Basin and the influence of this mixing activity on the flow dynamics and water mass conversion at the Reykjanes Ridge has never been investigated.

Ojectives: In the RREX project, we propose an in-depth investigation of the interactions of the ocean currents with the Reykjanes Ridgein order to: document the circulation around and over the ridge and identify the processes controlling the dynamical connections between the two sides of the ridge; quantify and understand the water mass transformation near the Reykjanes Ridge; identify key parameters in general circulation models that are critical for an adequate representation of the circulation and water mass transformation at the Reykjanes Ridge and provide recommendations for the improvement of the ocean component of the next generation of climate model. To achieve those objectives, laboratory experiments and idealized simulations combined with a new and ambitious dataset and new realistic model simulations will be used altogether.

Schematic of the horizontal circulation in the North-Atlantic Ocean. The surface/subsurface circulation, which transports the warm and salty waters of subtropical origin of the upper limb of the MOC, is in red/orange. IC and NAC: Irminger Current and North-Atlantic Current. The deep currents transporting the cold and less salty waters of the lower limb of the MOC are in blue. ISOW and DWBC: Iceland Scotland Overflow Water and Deep Western Boundary Current. The green arrows represent cold and fresh surface coastal currents: the Labrador, East and West Greenland Currents (LC, EGC, WGC).The. Labrador Sea Water (LSW) spreading is represented in purple. The Charlie Gibbs Fracture Zone (CGFZ) is at the southern tip of the Reykjanes Ridge near 52°N. The northward Irminger Current (IC) flows on the western side of the Reykjanes Ridge. Position of the OVIDE (black dots), AR7E (yellow dots), and 60°N (green dots) sections are displayed as well as the moorings deployed by Van Aken (1995) (magenta diamond) and by van Haren (2007) (black diamond).

Position of the CTDO2 casts (red dots). VMP
measurements will be done every other CTD cast. The
yellow dots on section BC and DE indicate the position
of the two ASFAR devices that will release 1 float every
3 months. The black points indicate the moorings

Mean 2002-2010 circulation along the Ovide line. Positions of the seven moorings are superimposed. The moorings are equipped with T/S recorder (black dots), upward looking ADCP (red dots), current meters (green dots), thermistor strings
(blue dots).